On Tue, Oct 10, 2006 at 06:46:08PM -0700, Geoff Levand wrote: > > Linas Vepstas wrote: > >> The current driver code performs 512 DMA mappns of a bunch of > >> 32-byte structures. This is silly, as they are all in contiguous > >> memory. Ths patch changes the code to DMA map the entie area > >> with just one call. > > Linas, > > Is the motivation for this change to improve performance by reducing the > overhead > of the mapping calls?
Yes. > If so, there may be some benefit for some systems. Could > you please elaborate? I started writingthe patch thinking it will have some huge effect on performance, based on a false assumption on how i/o was done on this machine *If* this were another pSeries system, then each call to pci_map_single() chews up an actual hardware "translation control entry" (TCE) that maps pci bus addresses into system RAM addresses. These are somewhat limited resources, and so one shouldn't squander them. Furthermore, I thouhght TCE's have TLB's associated with them (similar to how virtual memory page tables are backed by hardware page TLB's), of which there are even less of. I was thinking that TLB thrashing would have a big hit on performance. Turns out that there was no difference to performance at all, and a quick look at "cell_map_single()" in arch/powerpc/platforms/cell made it clear why: there's no fancy i/o address mapping. Thus, the patch has only mrginal benefit; I submit it only in the name of "its the right thing to do anyway". --linas - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
